I was always told to watch birds while fishing. From watching feeding birds to just seeing herons on the bank fishing. Baitfish are a food source for birds and bigger fish so it can't hurt to pay attention to that.
While deer hunting I have been told to pay attention to what the cows are doing. If they are feeding it could be good..if they are chilling then get ready for a long day. I don't have enough experience around cows to know if that's true??